start by defining variables (things you don't know). Let's make:
t = number of true/false questions
m = number of multiple choice questions
Equation #1
total questions on the test (given to be 20) is then:
t + m = 20
Equation #2
Multiply the number of questions by the points it's worth to get the total points
3t + 11m = 100
Now, you can use substitution to combine the two equations.
t + m = 20
by subtracting m from both sides, equation 1# becomes..
t = 20 - m
substitute t for 20 - m in the second equation.
3(20 - m) + 11m = 100
Solve for m.
3(20 - m) + 11m = 100
60 - 3m + 11m = 100
8m = 100 - 60
8m = 40
m = 5
There are 5 multiple choice questions on the test.
Now use this to find t.
t + m = 20
t + 5 = 20
t = 20 - 5
t = 15
there are 15 true/false questions on the test.